3-MeTHF has been produced in commercial quantities by the high pressure hydrogenation of citraconic anhydride and some of its derivatives according to the procedures disclosed in U.S. Pat. No. 5,536,854 and Published Japanese Patent Application (Kokai) 08-217,771. Since citraconic acid is formed from citric acid or, more economically, as a minor by-product, during maleic anhydride production, these routes to 3-MeTHF are expensive and use a starting material which is not plentiful.
Processes for the production of 3-MeTHF based on less expensive precursors and precursors independent of the production of other materials have been developed. Thus, U.S. Pat. No. 3,932,468, describes a process for isomerizing isoprene monoepoxide into 4-methyl-2,3-dihydrofuran using a nickel and hydrohalic acid catalyst. Although the hydrogenation of 4-methyl-2,3-dihydrofuran into 3-MeTHF is relatively simple, the synthesis of the starting material, isoprene monoepoxide, is not. For example, the preparation of isoprene monoepoxide would require the use of classical (and expensive) epoxide manufacturing techniques such as the use of halohydrins or co-oxidation with aldehydes. Japanese Published Patent Application (Kokai) JP 08-291,158 describes another method for preparing 3-MeTHF in which propylene is converted into 2-methylsuccinate esters by a double oxidative carbonylation in the presence of an alcohol. Although the reductive cyclization of the 2-methylsuccinate esters to 3-MeTHF is facile, the double oxidative carbonylation reaction usually gives limited yields of the dicarbonylated products and requires expensive, reactive solvents to keep the reagents anhydrous.
Another method for the synthesis of 3-MeTHF is disclosed in U.S. Pat. No. 3,859,369 and comprises the hydroformylation of 2-buten-1,4-diol into 2-methyl-1,4-butanediol which is converted to 3-MeTHF by acid catalysis. U.S. Pat. Nos. 4,590,312 and 4,879,420 describe the conversion of 4-hydroxybutyraldehyde and its immediate precursor, 2-buten-1,4-diol, into 3-MeTHF by reductive alkylation with formaldehyde followed by acid catalyzed cyclization. In each case, the products were mixtures of 3-MeTHF and tetrahydrofuran. This situation occurred in the hydroformylation process because isomerization accompanied the hydroformylation, limiting the yield of 3-MeTHF by forming a tetrahydrofuran precursor. In the reductive alkylation processes, the intermediate products as well as the starting materials could form alcohols by hydrogenation. Only those hydrogenations occurring after an initial aldol condensation of the reactants with formaldehyde could form 3-MeTHF. All other hydrogenations gave tetrahydrofuran or other byproducts.
The preparation of 3-MeTHF also is disclosed in Published European Patent Application EP 727 422 and involves the hydrocyanation of methacrylate esters. A series of hydrolyses and esterifications forms a diester which may be reductively cyclized to 3-MeTHF using an acidic, copper chromite catalyst. In this case, not only were the starting materials expensive (although not as expensive as the citraconic anhydride derivatives), but the synthesis required four steps. Japanese Published Patent Application (Kokai) JP 08-217,708 describes a process for producing 3-MeTHF by the hydroformylation of methacrylate esters to form mixtures of the .alpha.-formylisobutyrate and the .beta.-formylisobutyrate esters using synthesis gas. Japanese Published Patent Application (Kokai) JP 08-217,770 discloses a similar hydroformylation using methyl formate as the C-1 source. In both of these hydroformylation processes, hydrogenation of the resulting .beta.-formylisobutyrate ester over a copper chromite catalyst gave 3-MeTHF. One further hydroformylation route reported in Published European Patent Application Publication EP 747,373 consists of (1) the hydroformylation of isobutenyl alcohol (2-methyl-2-propen-1-ol) to form 4-hydroxy-3-methylbutyraldehyde which (2) was readily hydrogenated with nickel catalysts to 2-methyl-1,4-butanediol and which (3) was cyclized to 3-MeTHF by acid catalysis.
U.S. Pat. No. 5,856,527 discloses a process for the preparation of 3-alkyltetrahydrofurans by a two-step process wherein 2,3-dihydrofuran is reacted with an acetal to form an intermediate compound which may be converted to a 3-alkyltetrahydrofuran by contacting the intermediate with hydrogen in the presence of a catalystic amount of a Group VIII noble metal or rhenium and a strong catalyst. U.S. Patent discloses a two-step process wherein (1) 2,3-dihydrofuran is reacted with a trialkyl orthoformate in the presence of an acidic catalyst to produce 2-alkoxy-3-dialkoxymethyl)-tetrahydrofuran and (2) the intermediate is contacted with hydrogen in the presence of a catalyst system comprising a Group VIII noble metal or rhenium and a strong acid to convert the intermediate to a mixture of 3-MeTHF and 3-HOMeTHF.
U.S. Pat. No. 5,912,364 discloses the preparation of 3MeTHF by contacting 3-formyltetrahydrofuran with hydrogen in the presence of a catalyst system comprising a Group VIII noble metal or rhenium and a strong acid under hydrogenolysis conditions of temperature and pressure. The disclosed process typically produces a mixture of 3-MeTHF and 3-HOMeTHF. This patent also discloses processes for the preparation of 3-formyltetrahydrofuran by contacting 2,5-dihydrofuran with synthesis gas comprising carbon monoxide and hydrogen in the presence of a rhodiumphosphorus catalyst system according to known hydroformylation procedures. U.S. Pat. No. 5,945,549 discloses a process for the recovery of formyltetrahydrofurans (FTHF's) produced by the rhodium-catalyzed hydroformylation of 2,3-dihydrofuran wherein the FTHF's are recovered as an equilibrium mixture of 2- & 3-FTHF and their hydrates, 2- and 3-[di(hydroxy)methyl]tetrahydrofuran from a hydroformylation product solution comprising a rhodium catalyst, 2- and 3-FTHF and an organic hydroformylation solvent obtained as a liquid product take-off from a hydroformylation process wherein 3-FTHF is produced by the hydroformylation of 2,5-dihydrofuran. These known methods for the production of 3-MeTHF starting with 3-formyltetrahydrofuran suffer from one or more disadvantages such as low reaction yields, the co-production of other compounds which have limited utility and/or the use of corrosive aicds.